Integrated circuits are generally subjected to a test process in order to verify their correct operation; this is important to ensure high quality of a manufacturing cycle of the integrated circuits. The test process may be aimed at identifying defects that are either evident or potential (i.e., which might occur after a short life of the integrated circuits). In the latter case, the integrated circuits are commonly tested under stress conditions; a typical example is the burn-in test, which consists of making the integrated circuits work for some hours at very high or very low temperature (such as ranging from −50° C. to +150° C.), in order to simulate a long period of operation of the same integrated circuits at room temperature (i.e., 25° C.-50° C.).
The test process may be performed when the integrated circuits are still on a wafer of semiconductor material (where a high number of identical integrated circuits are formed in different areas thereof). In this way, it is possible to save the costs of any next manufacturing steps (such as for the packaging) when the integrated circuits are defective; moreover, a high number of integrated circuits included in each wafer may be tested concurrently. In some cases, it is also possible to fix certain defects of the integrated circuits (so as to avoid their rejection).
Typically, the test process is implemented by means of a prober. The prober interfaces with the wafer by means of a probe card, which is provided with multiple probes for contacting corresponding terminals of the integrated circuits electrically. For this purpose, the wafer is locked on a chuck plate (for example, by means of vacuum grooves). The prober moves the chuck plate until the wafer is aligned with the probe card (so that each terminal of the integrated circuits is coupled with a corresponding probe); the prober then applies stimulus signals to the integrated circuits and receives corresponding response signals (through the probes). Particularly, availability of a full wafer probe card allows testing all the integrated circuits of the wafer at the same time. However, once the wafer has been aligned with and contacted by the (full wafer) probe card, the prober remains idle for the duration of the entire test process of the wafer (typically several hours), but at the same time completely unavailable for any other operation.
In order to solve this problem, WO-A-01/04643, which is incorporated by reference, proposes the use of a test cartridge. Particularly, the probe card is mounted in a movable manner on a corresponding plate. A mechanical system locks the chuck plate and the probe plate together (for example, by means of a pair of jaws). The position of the probe card with respect to the chuck plate (and then to the wafer) is controlled by means of a pneumatic system. For this purpose, in a proposed implementation an O-ring defines a sealed chamber between the chuck plate and the probe card; holes are formed in the probe card for controlling an air pressure in the chamber, so as to advance or retract the probe card with respect to the chuck plate. In this way, the combined actions of the mechanical system and the pneumatic system allow obtaining the desired pressure between the probe card and the wafer.
In operation, the prober aligns the wafer with the probe card and then locks them in the correct position (by means of the above-described mechanical and pneumatic systems). The cartridge is then removed from the prober so at to make it available for assembling another cartridge. A batch of cartridges so obtained is then provided to a distinct test machine. Particularly, the cartridges are placed into a burn-in chamber, and they are connected electrically to a test circuit (outside the burn-in chamber) by means of corresponding connectors arranged on the probe plates. During the test process, the temperature of the wafers is controlled by means of a heating or cooling fluid that circulates through channels formed in the chuck plates. This allows exploiting the (very expansive) mechanical section of the prober at its best—since it is used only for the assembling of the cartridges; moreover, it is now possible to test more wafers concurrently in the same test machine.
However, the structure of the cartridge disclosed in WO-A-01/04643 (and especially its mechanical system for locking the chuck plate and the probe plate together) is relatively complex and difficult to actuate. Moreover, each cartridge includes a dedicated chuck plate (required to couple the wafer with the probe card by means of the above-described mechanical and pneumatic systems); this has a detrimental effect on the cost of the cartridge.
An alternative solution is disclosed in US-A-2002/0003432, which is incorporated by reference. In this case as well, a plurality of cartridges are assembled off-line in a prober, and then provided to a distinct test machine. However, each cartridge is now obtained by locking the probe card and the wafer with a vacuum clamp. Particularly, the clamp is formed by a first mechanical member that is closed around a back surface of the probe card (opposite the one with the probes) through a first seal, and a second mechanical member that is closed around a back surface of the wafer (opposite the one with the terminals) through a second seal; the two mechanical members are coupled by means of a third seal. The clamp has a port, which is used to create the vacuum in a region defined by the probe card, the wafer and the clamp (so as to obtain the desired pressure between the probe card and the wafer). A mechanical latch (such as a wing nut) maintains the clamp tightened (even if vacuum is lost during transport of the cartridge). Springs may be associated with the probes to remove heat from a predefined internal region of the wafer where the integrated circuits are formed (thereby directing the cooling away from its peripheral region, which generates little or no heat); moreover, this structure leaves access to most of the back surface of the wafer for cooling (with the exception of the part covered by the clamp).
However, the above-described cartridge may generate unwanted bowing at an edge of the wafer; indeed, as pointed out in the same document, in order to avoid this problem the probes should extend to an edge of the probe card or the seal (between the clamp and the wafer) should extend to a last row of probes. Moreover, the structure of this cartridge remains relatively complex and difficult to actuate; particularly, its assembling requires a number of mechanical actions to mount the clamp and to tighten the latch. In any case, the solution disclosed in US-A-2002/0003432 cannot be implemented in standard probers (wherein the wafer is aligned with the probe card by acting on the chuck plate); indeed, in this case an optical alignment system based on glass reticles provided in the probe card is required.
All of the above maintains the overall cost of the test process relatively high; this drawback limits the use of the test process, and accordingly lowers the level of quality and reliability in the production of the integrated circuits.